In the field of high intensity (HID) lamps, the high pressure mercury and the metal halide families generally utilize a quartz or fused silica arc tube enclosed within a glass outer jacket fitted with a screw base at one end. In the mercury lamp the arc tube contains a filling of mercury whereas in the metal halide lamp the filling comprises mercury and metal halides. In both kinds, the inner arc tube transmits ultraviolet radiation which is absorbed without harm by the glass outer envelope, or even absorbed gainfully by a phosphor coating on the outer envelope.
In most lamps the outer envelope remains intact to the end, and life is ended by other factors. However, its does happen occasionally that the outer envelope or jacket is punctured or shattered while the arc tube remains intact and the lamp continues to operate. In this mode of operation, ultraviolet radiation from the arc tube may escape and create a safety hazard. The Bureau of Radiological Health of the Food and Drug Administration (Dept. of HEW) has requested lamp manufacturers to develop self-extinguishing lamps, that is lamps with built-in safeguards that will permanently extinguish the arc discharge in the event of jacket failure.
One type of self-extinguishing lamp achieves self-extinction by means of a mechanical switch connected in series with the arc tube and maintained closed by pressure from contact with the jacket. While such a switch is relatively inexpensive, instantaneous in its action and wastes no power, it may not be activated if the jacket is merely punctured and not completely broken away. Published FDA Regulations (Fed. Reg. Vol. 44, No. 175, p 52191-6) coming into force in September 1981, require that a self-extinguishing lamp cease operation within a cumulative operating time not exceeding 15 minutes following removal of at least 3 square centimeters of contiguous surface of the outer envelope. The mechanical switch safety lamp will not meet this specification.
To comply with the new F.D.A. requirement, it is necessary to go to designs utilizing an oxidizable fuse connected in series with the arc tube and located within the outer envelope. It has been proposed to use a tungsten or a molybdenum filament which should have a low resistance in order to minimize power dissipation and also assure a filament life matching that of the relatively long-lived arc tube. However, a low resistance filament may require an excessively long time to oxidize and open the circuit after outer envelope breakage. In U.S. Pat. No. 4,013,919 - Discharge Lamp Having Fuse Switch Guard Against Jacket Failure, a filament serving as a fuse is provided in series with the arc tube and a thermal switch shunts the filament. This permits a filament of higher resistance to be used, and wastage of energy is prevented because heat from the arc tube causes the thermal switch to close and short-circuit the filament under normal operating conditions. While a filament fuse shorted by a thermal switch can satisfy the FDA specification, it is a complicated and relatively costly solution.